more unique filename for test

I actually had a filename named /tmp/foo and this test failed, because
it was owned by a different user, hence writing to it (and deleting it)
failed. Using /tmp/foo_iptest makes it a bit more unique, and a lot less
likely to be on a given system.
Fix doc: "principle" not "principal"
Merge pull request #3413 from ivanov/foo_iptest

more unique filename for test
Merge pull request #3416 from jzwinck/patch-1

Fix doc: "principle" not "principal"
thansk -> thanks
Merge pull request #3420 from MercuryRising/patch-1

thansk -> thanks
process escapes in mathjax

allows  to enter dollar signs in markdown.
Merge pull request #3428 from minrk/escapeDollar

process escapes in mathjax

\$ can now be used to insert dollar signs in markdown cells
Merge remote-tracking branch 'upstream/master'